SAW filters have been used to allow passage of a narrow range of frequencies (bandwidth) while sharply attenuating all frequencies outside that bandwidth. The center frequency of SAW filters varies with temperature, ageing and unit to unit variations. Therefore, in order that the desired frequency always is allowed to pass, the SAW filter must have a substantial bandwidth. This requires more circuit area and greater cost. The resonant frequency of SAW resonators also varies with temperature, ageing and unit to unit variations. The resonant frequency of a SAW resonator is a single frequency and not a band of frequencies. SAW resonators could not be used as filters because the desired frequency could suffer attenuation if the resonant frequency of the SAW resonator departs from the desired frequency. SAW resonators are used to make oscillators with the SAW resonator stabilizing the oscillation frequency. The oscillator must be able to tolerate the small frequency shift of the resonant frequency of the SAW resonator over temperature, time and unit to unit variation